Spong, in a copending application entitled "Information Record and Related Recording and Playback Apparatus and Methods", U.S. Pat. No. 4,097,895, filed Mar. 19, 1976, and incorporated herein by reference, describes an ablative recording system whereby a focussed modulated light beam, such as a laser beam, is directed at an ablative recording medium. The recording medium comprises a light reflecting material coated with a light absorbing material on a substrate. The thickness of the light absorbing layer is chosen to reduce the reflectivity to a minimum value, so that a maximum of light energy impinging on it is retained therein and is converted to thermal energy. This thermal energy causes the light absorbing material in the area struck by the light to sublime or melt thereby exposing selected portions of the light reflecting layer. During readout, the contrast between the light reflected from the absorbing layer which is at the reflection minimum and the light reflecting layer is detected.
Ongoing work in this area has resulted in the improved performance of materials employed. Thus, in an illustrative embodiment of this recording medium, a substrate which is a flat, smooth non-conductor of heat is coated with a thin layer of a light reflecting material such as aluminum. The aluminum layer is passivated as described in a copending application entitled "Ablative Optical Recording Medium" by Bartolini et al, Ser. No. 668,504, filed Mar. 19, 1976. The passivated aluminum layer is in turn coated with a layer of an organic light absorbing material such as 4-phenylazo-1-naphthylamine as described in Bloom et al, "Ablative Optical Recording Medium", U.S. Pat. No. 4,023,185.
Alternatively, the light reflecting layer is coated with a transparent dielectric material such as silicon dioxide. A thin layer of a metal is coated thereon to serve as the light absorbing layer. This configuration is described in the copending application of Bell entitled "Information Record", Ser. No. 782,032, filed Mar. 28, 1977, now abandoned. Metals which have been used for the light absorbing layer include titanium and Inconel, an alloy of 65-80 weight percent of nickel, 10-30 weight percent of chromium and 0-10 weight percent of iron.
When an organic dye is used as the light absorbing layer, a continuing problem is the mechanical delicacy of the dye layer. A protective coating with good mechanical properties would make it easier to handle the recording medium without damage to the dye layer. Another problem affecting both configurations of the recording medium is the effect of surface dust in causing signal defects or dropouts. When a recording is made on a freshly prepared optical video disc and a playback frame is displayed on a television monitor, fewer than ten signal defects or dropouts are observed. Similar recordings made several days later on the same disc show about 100 defects. The number of defects increases progressively as the disc ages. This is undesirable for commercial applications.
The rising defect count is predominantely due to the accumulation of many small dust particles, most of which are less than 10 microns in size, which precipitate from the environment onto the surface of the disc. As each dust particle moves under the focussed spot of the recording laser beam, it effectively shadows that portion of the track on which it lies, thus preventing the formation of information pits for that segment of the picture signal. On playback the section of recorded track which contained the dust particle exhibits a picture defect or dropout caused by the temporary loss of information. An improved recording medium would make it possible either to remove dust safely or to record without increased defects despite the presence of surface dust.